Hydroxyl radical (HO*) is one of the strongest oxidants that is available in the natural world and in the chemist's tool box. This species is extremely reactive and degrades promptly any available organic molecule. Hydroxyl radicals, likewise other reactive oxygen species, are also a very potent biocide which vigorously attack microorganisms and tissues, damaging cell membrane lipids, proteins, carbohydrates and DNA. The hydroxyl radical reactions are extremely fast, having rate constants of from 107 to 109 M−1 sec−1. Consequently the hydroxyl radical only diffuses the equivalent of 5-10 of its molecular diameter before it reacts [E. Cadenas: Ann. Rev. Biochem. 58 (1989) 79]. Interestingly, hydroxyl radicals and other reactive/oxygen species were recently recognized as a cellular signaling device [E.g., Van Breusegem et al.: Plant Sci. 161 (2001)405-414], as well as a biological weapon used by the immune system for killing bacteria [Wentworth P. et al.: Science 298 (2002) 2195-9].
Hydroxyl radicals injure cell membrane lipids via the following chain mechanism:    a) Detachment of a hydrogen radical randomly from a fatty acid, leading to the formation of an alkyl radical:RH+HO*→R*+H2O    b) The alkyl radical reacts instantly with oxygen (which is available in aerobic environment) to yield an alkylhydroperoxide derivative:R*+O2→ROO*    c) The alkyl hydroperoxide reacts with water to generate an alcohol and a hydroperoxyl radical:ROO*+H2O→ROH+HOO*
The transformation of a fatty acid skeleton into an alcohol renders the destruction of the membrane structure and consequently the collapse of the cell wall and the death of the organism.    d) The hydroperoxyl radical absorbs a hydrogen radical from another fatty acid molecule to yield an alkyl radical that subsequently reacts with oxygen, as shown in step b):RH+HOO*→R*+H2O2 
The hydrogen peroxide formed becomes the source of additional hydroxyl radicals which renew the cycle. This chain process propagates until a termination step such as the following one takes place:2ROO*→non-radical oxygenated products.
A fundamental feature of hydroxyl radicals (and other reactive oxygen species) is the benign nature of their decomposition products. No chemicals are formed as a result of their application in various water treatments but water.
Several methods for the generation of hydroxyl radicals are known, based mainly on hydrogen peroxide as the source. The simplest and the most accepted methodology is based on the Fenton reaction where ferrous ion reacts with hydrogen peroxide in an acidic media as follows [Legrini O.: Chem. Rev. 93 (1993) 671-98]:Fe+2+H2O2→Fe+3+HO*+OH−
This reaction is made catalytic predominantly under the effect of UV irradiation of the wavelength greater than 300 nm, which allows the recycling of Fe+3 to the original Fe+2, which process is named “Photo-Fenton” reaction. The main drawback of the Fenton type procedures is their strong dependence on pH. Thus the above reaction occurs only under acidic conditions, preferably at pH=2.5-3.0.[Andreozzi R. et al.: Catalysis Today 53 (1999) 51-9]. Consequently the Fenton scheme is not practicable under the numerous situations where acidic media cannot be used, such as in ballast water treatment, for example.
Another procedure for the formation of hydroxyl, and hydroperoxyl, radicals uses photocatalysis together with semi-conductors such as TiO2. Upon irradiation of TiO2 particle by the light having wavelength about 315-395 nm, an electron is ejected, generating a “hole” on the particle surface:TiO2+hν→TiO2(h+)+e−The “hole” interacts with water to produce a hydroxyl radical:TiO2(h+)+H2O→TiO2+HO*+H+
The free electron reacts with oxygen molecule to produce an anion of superoxide radical which is protonated to yield a hydroperoxyl radical:e−+O2→O2−*O2−*+H+→HOO*
Another widespread technique for the making of hydroxyl radical is direct photolysis of hydrogen peroxide under UV irradiation:H2O2+hν→2 HO*
This procedure suffers from low efficacy due to the very low molar extinction coefficient of the hydrogen peroxide molecule (about 19 dm3.mol−1.cm−1 at 254 nm at neutral pH [Baxendale J. H. . : Trans. Faraday Soc. 53 (1957) 344].
Using EPR techniques, Giamello E. et al. [J. Phys. Chem. 97(1993) 5735-40] showed that reactive oxygen species were formed upon contact of hydrogen peroxide with the surface of magnesium oxide in an aqueous environment. These authors also observed a highly unexpected stability of hydroxyl radicals adsorbed on the MgO surface, wherein the radicals were stable at temperatures as high as 200° C.
It is an object of this invention to provide a high efficacy method for producing hydroxyl radicals in aqueous environment using UV irradiation of hydrogen peroxide solutions.
It is another object of this invention to provide a method for producing hydroxyl radicals in water solutions of hydrogen peroxide and oxygen, combining UV irradiation with a catalytic effect of magnesium oxide.
Other objects and advantages of present invention will appear as description proceeds.